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A finding of vapor-dominated zones within the caprock beneath the major fumarolic manifestations and its implications; CSAMT survey of fumarole area in Hakone volcano, Japan

Kazutaka Mannen1 , Ryosuke Doke2, Akira Johmori3, George Kikugawa1, Takuto Minami4, Tetsuya Takahashi3, Mitsuru Utsugi5, Koichiro Fujimoto6

  • Affiliations: 1Hot Springs Research Institute of Kanagawa Prefecture 2Graduate School of Science and Technology, Hirosaki University 3Neo Science Co. Ltd. 4Department of Planetology, Graduate School of Science, Kobe University 5Aso Volcanological Laboratory, Institute for Geothermal Sciences, Graduate School of Science, Kyoto University 6Tokyo Gakugei University

  • Presentation type: Talk

  • Presentation time: Thursday 14:15 - 14:30, Room R290

  • Programme No: 3.8.5

  • Theme 3 > Session 8

Abstract

Monitoring fumarolic areas is essential for detecting volcanic unrest and predicting phreatic eruptions, but interpreting the signals obtained requires understanding the underground structure. We implemented a CSAMT survey in Owakudani, the largest fumarole field on Hakone volcano, Japan. The caprock detected as a low-resistivity zone forms a plateau-like convexity with a diameter of approximately 500 m. The ground surface above the caprock top forms a forested area with patchy steaming areas, contrasting with the barren 2015 hydrothermal eruption center east of the region. The caprock top had subsided before the 2015 eruption, possibly because of the depressurization beneath the caprock. Interestingly, the caprock has local yet distinctive high-resistivity zones, particularly beneath major fumaroles and hot springs. These local high-resistivities and beneath the caprock are interpreted as vapor-dominated. Previous studies indicate that meteoritic water heated by steam from the vapor-dominated hydrothermal system forms hot springs in the fumarole area. However, our chemical analysis indicates that merely mixing the meteoritic water with the steam beneath the caprock of Owakudani, which is high-Cl and acidic, cannot produce surface hot spring waters there. The two vapor-dominated zones beneath the major hydrothermal manifestations likely differentiate the hydrothermal fluid.